The present invention relates to an optical focus sensor system.
Optical focusing techniques are, of course, well known in the prior art. In applications where focusing requirements are in the micron range, numerous parameters are involved, such as the stability of the optics or lens used in the system, the rigidity and stability of the lens support and the object and image planes, temperature and vibration, among others. Any of the above parameters can dramatically affect the proper focusing of the system.
Some prior art approaches in automatic focusing systems have included some form of arrangement in which the amplitude of a signal corresponding to the proximity of best focus of an image (such as an image of a semiconductor reticle) is utilized for focusing techniques. As an example, such an approach is described in U.S. Pat. No. 3,781,110. Such prior art approaches in general depend upon an object having some spatial frequency components equal to that of a grating in an image plane. While prior art approaches can provide an indication of the proper location of a desired focal plane, there is no capability of indicating with a single measurement how far and in what direction the proper or desired focal plane is located.